1. Field of the Invention
The present invention relates to a plasma arc torch having a cover, an electrode, body parts and a nozzle having an orifice.
2. Description of the Related Art
In a plasma arc torch the main arc utilized for welding is excited between the torch electrode and the workpiece. The nozzle section of the torch is comprised of two coaxial cavities. The inner cavity houses a tungsten electrode and the end of the cavity is provided with an orifice about the tip of the electrode. A plasma gas is fed into this cavity. The inner cavity is enclosed by another cavity whose exit orifice surrounds the exit hole of the inner cavity. The shielding gas which envelops the electric arc is fed to this outer cavity.
Because the electric arc of the plasma torch is maintained in a gas atmosphere between the workpiece and the electrode, the gas must be ionized before the ignition of the main arc in order to make the gas electrically conductive. This ionization is accomplished by means of a pilot arc excited between the electrode and the nozzle that forms the inner cavity. The pilot arc ionizes the plasma gas, whereby a conductive ionized gas path is formed between the workpiece and the electrode, thus providing proper ignition conditions for the main arc.
The main arc must be maintained only between the electrode and the workpiece, because such a high-energy electric arc so between the electrode and the nozzle would rapidly destroy the nozzle. Normally, the cooling of the nozzles and the electrical and magnetic forces acting in the nozzle prevent the main arc from being excited between the electrode and the nozzle. This requires, however, that the electrode tip must be exactly aligned to the electrical center point of the nozzle. If the nozzle orifice and the electrode tip have symmetrical shapes, the electrical center point generally also coincides with the geometric center point.
The shape of the nozzle orifice and thus the position of the electrical center point may change during welding for several reasons: The electrode tip may be offset from the electrical center point already from the start of welding due to unavoidable production tolerances of the torch, nozzle and electrode. Resultingly, the position of the nozzle orifice undergoes slow shifting during welding, causing the plasma jet to deviate. The orifice shape itself may often also become deformed due to welding splashes and accumulation of other debris. When the plasma jet direction diverges, working with the plasma torch becomes difficult and finally impossible. The welding seam quality worsens and repeatability will be lost due to the varying behavior of the plasma arc. The pilot arc weakens, and the ignition of the main arc becomes more difficult so that finally the main arc cannot be ignited at all. At this stage the nozzle and generally the electrode as well must be replaced. As nozzles in plasma torches are easily damaged, nozzle changes become the cause of frequent interruptions in welding operations, which thus are hampered by the high consumption rate of the nozzles.
In plasma torches intended for manual welding, altering the position of the electrode is possible only through machining of the electrode tip, because the electrode is permanently aligned with respect to the torch body by means of ceramic support pieces. Reshaping of the electrode is a slow and time-consuming operation, since the work must be done in a machine due to the high tolerance requirements.
In larger plasma torches used in mechanically controlled welding, the electrode position can be adjusted with the help of an eccentric mechanism. These torches have a large-diameter nozzle orifice, and the main arc is ignited by means of a high-frequency arc which rotates in the gap between the electrode and the nozzle. The centering of the electrode is accomplished by first igniting the high-frequency arc and then aligning the electrode with the help of the eccentric mechanism until the arc starts to rotate about the nozzle orifice in a symmetrical manner. Such a mechanism is, however, too massive for hand-held torches and can be used only in torches ignited by a high-frequency arc. The electrode position is not adjustable by such an arrangement after the main arc has been ignited, because the torch is not gas-tight during the adjustment.